The present invention relates to a veneer dehydrating apparatus having a pair of rotatable rolls through which a sheet of green, or not dried, wood veneer is passed to be compressed for squeezing part of water contained in the veneer sheet. More specifically, it relates to an apparatus which can be used advantageously for dehydrating coniferous or softwood veneer.
The purpose of veneer dehydration is to remove part of water contained in sheets of as-peeled green veneer, which is yet to be dried, for reducing its initial moisture content, thereby shortening the time required for the subsequent kiln drying of such sheets to the desired level of moisture content. A typical veneer dehydrating apparatus is disclosed by Publication of Examined Japanese Patent Application No. 52-9721 (1977). This apparatus includes a pair of rolls disposed one above the other, extending parallel to each other and spaced apart radially such that the veneer sheet is compressed by the rolls by about 30 to 60 percent of the sheet thickness as the sheet is being passed through the rolls. That is, the paired rolls are spaced apart from each other so as to provide a clearance between the peripheral surfaces thereof, whose distance in radial direction of the rolls corresponds to about 40 to 70 percent of the nominal thickness of veneer sheet to be processed. At least one of the rolls is formed on its periphery with a number of projections or ridges each extending in axial direction of the roll and having a height corresponding to about 50 percent or less of the veneer sheet thickness and a blunt or substantially flat tip face with a width of about 1 mm or less as measured across the axial extension of the ridge.
With this apparatus, a sheet of green veneer is fed toward the rolls with the general wood fiber orientation of the veneer directed perpendicularly to the sheet feeding direction and passed through the rolls. The veneer sheet is then compressed by the peripheral surfaces of the paired rolls by about 30 to 60 percent of its original thickness for squeezing part of water contained in the veneer sheet. In processing a veneer sheet of softwood with this apparatus, however, the veneer itself tends to be more readily deformed plastically or even broken when it is subjected to compression by about 30% or more of its original sheet thickness, with the result that the thickness of the dehydrated veneer sheet is reduced considerably. If a plurality of such plastically deformed veneer sheets laid up in a stack is hot-pressed for gluing in the manufacturing of plywood or LVL (laminated veneer lumber) panels, the resulting product may fall below a lower permissible limit of the desired product thickness, thus degrading its quality.
Another problem derives from the fact that a softwood veneer sheet contains many knots extending across its thickness and such knots tend to be broken readily into small pieces when the sheet is compressed by about 30 percent or more. Such wood pieces produced by broken knots may cause a trouble in the subsequent processes in handling of veneer sheets and making of plywood or LVL panels. As a matter of course, the resulting panel product having therein a void due to a broken knot will degrade its quality and limit the usage. Thus, the above conventional apparatus for dehydrating veneer by compression by rolls is not practically applicable to dehydrating softwood veneers.
If the dehydrating rolls of the apparatus are spaced so as to provide a larger space, for example about 75 percent or more of veneer sheet thickness, the above problems may be avoided, but satisfactory dehydration effect cannot be achieved because of insufficient compression of the sheet.